1. Field of the Invention
This invention relates to the field of electrical instrumentation and control, and more particularly, to an instrumentation apparatus which enables the sensing of physical parameters and the control of operational functions in a remotely located A.C. powered installation.
2. The Prior Art
Numerous industrial installations exist in which an A.C. powered motor and pump assembly or other alternating current device is operated at a remote location to which access is difficult, costly and/or impractical, if not impossible. An example of such an installation is the motor and pump assembly of a submersible pumping system operating near the bottom of a deep bore or hole in the ground (referred to as "down-hole"). In such installation, there is often a requirement to monitor certain physical parameters present in the down-hole environment, particularly the temperature and pressure therein. Moreover, in such installations, the requirement to remotely control certain operational functions, such as the flow of fluid through a down-hole, solenoid controlled valve also arise.
In some systems of the prior art, sensors are installed in the down-hole portion of the installation and electrically coupled to readout and recording equipment located at the surface, through instrumentation wiring, separate from the cable wiring which conducts power to the down-hole A.C. motor. Experience has shown, however, that such separate instrumentation wiring is unreliable; i.e., that it is fragile and easily damaged, particularly in a down-hole pumping environment. To overcome this problem, some systems of the prior art utilize the wiring of the power cable to carry instrumentation and control signals between the surface equipment and the down-hole or remotely located sensors (or control devices). This approach eliminates the problem of fragile instrumentation wiring in that the power cable is typically rugged and reliable. Examples of such prior art systems are disclosed in U.S. Pat. Nos. 3,284,669 and 3,340,500 issued to C. A. Boyd and C. A. Boyd et al respectively.
Prior art systems which utilize the power cable for transmission of instrumentation and control signals, however, have one significant shortcoming attributable to the fact that the instrumentation apparatus is permanently coupled to the power cable. This shortcoming arises in those applications where it becomes necessary to test the A.C. operating device, e.g., the down-hole motor, and the power cable at high voltage levels. In a submersible pumping installation, the test voltages may be as high as 2500 volts D.C. or RMS. In such application, the permanent coupling of the down-hole instrumentation package to the power cable prevents the conduct of such high voltage testing because either (i) the sensors in the instrumentation package cannot withstand the high voltages or (ii) the sensor circuits therein present a low impedance path to ground, thereby precluding the conduct of meaningful tests of the motor and power cable.
The present invention overcomes the foregoing limitation of the prior art by providing remotely controlled switching means for coupling and decoupling the down-hole (remote) instrumentation package to and from the power cable and the down-hole (remote) motor or other A.C. powered operating device. Thus, the present invention obtains the advantage of transmitting instrumentation and control signals over the power cable while, at the same time, not precluding high voltage testing of the motor and cable. Moreover, this invention contemplates utilizing the same switching means to selectively switch between multiple sensor and control circuits within the remotely located instrumentation package.
While down-hole sensing circuits, coupled to surface equipment through a power cable, are known in the prior art, no system heretofore has successfully combined the sensor and control circuits and switching means disclosed by this invention. Furthermore, the particular sensor and control circuits, and the particular switching means disclosed herein are not taught by the prior art. More specifically, this invention discloses a combination of (i) particularly advantageous latching and delatching circuits to control and operate the switching means, and (ii) a sensor circuit which, by a configuration of diodes, directs a sensor readout current through a path which first by-passes the sensor, in a "reference" mode, and by reversing the direction of such current, directs the same through a sensor, in a "measurement" mode. Since each sensor used is of the type which changes its resistance in response to a parameter of interest, the above-described sensor circuit eliminates measurement errors due to changes or drifts in the resistances or impedances of the various circuit components, cable and other conducting means in the readout current path. This is so because the present invention does not make an absolute measurement of a sensor's resistance, but rather, a measurement of the difference between the resistance of the entire readout current path with and without the sensor.
While the prior art shows down-hole switching means, such as the rectifier bridge and relay disclosed in U.S. Pat. No. 2,915,697 granted to E. S. Cornish, it does not show the particular combination of sensing, control and switching means disclosed herein.
An additional advantage of the present invention is that it is capable of electrically interfacing with motors of any horse-power and operating voltage. For example, the described apparatus may be employed, without alteration, with motors operating at 400 to 3300 volts (A.C.) and with currents in the range of 15 to 150 amps per phase.